U.S. patent number 10,359,653 [Application Number 15/840,602] was granted by the patent office on 2019-07-23 for thin-plate ln optical control device.
This patent grant is currently assigned to SUMITOMO OSAKA CEMENT CO., LTD.. The grantee listed for this patent is Sumitomo Osaka Cement Co., Ltd. Invention is credited to Katsutoshi Kondou, Eiji Murakami, Kiyotaka Nakano.
United States Patent |
10,359,653 |
Kondou , et al. |
July 23, 2019 |
Thin-plate LN optical control device
Abstract
A thin-plate LN optical control device includes: a thin-plate LN
optical waveguide element which includes an optical waveguide
formed by thermal diffusion of Ti in a substrate made of lithium
niobate, and a control electrode that is formed on the substrate
and is configured to control a light wave propagating through the
optical waveguide, and in which at least a part of the substrate is
thinned; and a housing that accommodates the thin-plate LN optical
waveguide element in an air-tight sealing manner. Oxygen is
contained in a filler gas inside the housing.
Inventors: |
Kondou; Katsutoshi (Tokyo,
JP), Nakano; Kiyotaka (Tokyo, JP),
Murakami; Eiji (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sumitomo Osaka Cement Co., Ltd |
Tokyo |
N/A |
JP |
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Assignee: |
SUMITOMO OSAKA CEMENT CO., LTD.
(Tokyo, JP)
|
Family
ID: |
62489217 |
Appl.
No.: |
15/840,602 |
Filed: |
December 13, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180164612 A1 |
Jun 14, 2018 |
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Foreign Application Priority Data
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Dec 14, 2016 [JP] |
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2016-241813 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F
1/035 (20130101); G02F 1/225 (20130101); G02F
1/0316 (20130101); G02F 1/2257 (20130101); G02F
1/03 (20130101); G02B 6/1342 (20130101); G02B
6/122 (20130101); G02F 1/0353 (20130101); G02B
6/13 (20130101); G02B 6/30 (20130101) |
Current International
Class: |
G02B
6/122 (20060101); G02F 1/225 (20060101); G02B
6/13 (20060101); G02B 6/30 (20060101); G02B
6/134 (20060101); G02F 1/03 (20060101); G02F
1/035 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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H07152007 |
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Jun 1995 |
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JP |
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2010085738 |
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Apr 2010 |
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JP |
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Other References
Hiroshi Nagata and Naoki Mitsugi, "Mechanical Reliability of LiNbO3
Optical Modulators Hermetically Sealed in Stainless Steel
Packages", Optical Fiber Technology, vol. 2, p. 216-224(1996).
cited by applicant.
|
Primary Examiner: Bedtelyon; John
Attorney, Agent or Firm: Greer, Burns & Crain, Ltd.
Claims
What is claimed is:
1. A thin-plate LN optical control device comprising: a thin-plate
LN optical waveguide element which includes an optical waveguide
formed by thermal diffusion of Ti, and a control electrode
configured to control a light wave propagating through the optical
waveguide in a substrate made of lithium niobate, and in which at
least a part of the substrate is thinned; and a housing that
accommodates the thin-plate LN optical waveguide element in an
air-tight sealing manner, wherein the substrate is thinned by
mechanical processing, the thickness of the substrate is 10 .mu.m
or less, and oxygen is contained in a filler gas inside the
housing.
2. The thin-plate LN optical control device according to claim 1,
wherein a molar concentration of oxygen in the filler gas is 3% or
greater.
3. The thin-plate LN optical control device according to claim 1,
wherein in the thin-plate LN optical waveguide element, a ridge is
formed in a front surface or a back surface of the substrate.
4. The thin-plate LN optical control device according to claim 1,
wherein the filler gas does not container moisture.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Patent Application
No. 2016-241813 filed Dec. 14, 2016, the disclosure of which is
herein incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an LN optical control device
including an LN optical waveguide element in which an optical
waveguide and a control electrode configured to control a light
wave that propagates through the optical waveguide are formed in a
substrate that is formed by using lithium niobate (LN), and a
housing that accommodates the LN optical waveguide element in an
air-tight sealing manner, and particularly to, an LN optical
control device (referred to as a thin-plate LN optical control
device) that uses the LN optical waveguide element that is
thinned.
Description of Related Art
In an optical communication field or an optical measurement field,
there is known an optical control device that uses LN having an
electro-optic effect capable of realizing high-speed response. As
one of LN optical control devices, there is known an LN optical
modulator in which an LN optical waveguide element is accommodated
in a housing. In the LN optical waveguide element, a Mach-Zehnder
type optical waveguide formed by thermal diffusion of titanium (Ti)
and a control electrode configured to control a light wave that
propagates through the optical waveguide are formed in an LN
substrate.
An optical bandwidth as an example of the performance of the LN
optical modulator has a trade-off relationship with a drive
voltage. As an example of a method of realizing an LN optical
modulator having low-drive-voltage performance in a relatively
broadbandwidth, there is known a method of thinning an LN substrate
in which the optical waveguide is formed or the LN optical
waveguide element (refer to Japanese Laid-open Patent Publication
No. 2010-85738).
The LN optical waveguide element is fixed to an inner side of the
housing by using an adhesive. In addition, an electrical
interconnection including the control electrode of the LN optical
waveguide element is formed by using a metal. When being left as is
in a moisture-containing atmosphere, an adhesive force of an
organic material such as the adhesive may deteriorate. In addition,
in a case where application of a voltage is performed continuously
in the moisture-containing atmosphere, disconnection or
short-circuiting due to migration may occur in the electrical
interconnection.
In addition, DC drift is known as a phenomenon peculiar to the LN
optical modulator. The DC drift is a phenomenon in which a voltage
(referred to as "control voltage") at an operation point for
driving the LN optical modulator varies with the elapse of DC
voltage application time when a DC voltage is continuously applied
to a control electrode of the LN optical modulator (refer to
Japanese Laid-open Patent Publication No. 7-152007).
To solve a problem related to reliability of the LN optical
modulator, the following structure is used. In the structure, an
atmosphere inside a housing, in which an LN optical waveguide
element is accommodated, is substituted with an inert gas (for
example, nitrogen, helium) that does not contain moisture, and the
housing is air-tightly sealed to block an atmosphere on an outer
side of the housing (refer to Hiroshi NAGATA and Naoki MITSUGI,
"Mechanical Reliability of LiNbO.sub.3 Optical Modulators
Hermetically Sealed in Stainless Steel Packages", OPTICAL FIBER
TECHNOLOGY, Volume 2, pages 216 to 224 (1996)).
The present inventors have prepared a thin-plate LN optical
modulator in which an LN optical waveguide element (thin-plate LN
optical waveguide element) that is thinned is accommodated in a
housing having an air-tightly sealed structure in which an
atmosphere inside the housing is substituted with an inert gas that
does not contain moisture. DC drift of the thin-plate LN optical
modulator has been evaluated. From the evaluation, when comparing
the DC drift with DC drift of an LN optical modulator that is not
thinned, it was confirmed that a DC drift amount (a fluctuation
amount of a control voltage after a constant time at predetermined
temperature and initial control voltage) increases, a variation in
a DC drift amount between samples increases, and reproducibility
(reproducibility of the amount and behavior of the DC drift when
measuring the DC drift a plurality of times by using the same
sample) is low.
In addition, with regard to an LN substrate, a crystal-grown boule
is sliced in a thickness of approximately several hundreds of .mu.m
to 1 mm, and a surface of the LN substrate is polished to be flat.
In a case where the thickness of the LN substrate is several
hundred .mu.m, a processing damage formed on the surface of the LN
substrate can be removed or recovered by chemicals or by thermal
annealing.
However, there is a concern that an LN substrate that is thinned to
several tens of .mu.m or less may be broken, and thus the LN
substrate is fixed to a reinforcing substrate by using an adhesive.
Accordingly, it is not easy to perform the above-described chemical
and thermal treatments. In addition, mass productivity is low under
mechanical processing conditions in which the processing damage
does not occur, and thus an inspection process is necessary.
SUMMARY OF THE INVENTION
An object of the invention is to solve a problem related to an
increase in a DC drift amount, a great variation between samples,
and low reproducibility which occur in a thin-plate LN optical
control device in which a thin-plate LN optical waveguide element
is accommodated in a housing, in which an atmosphere inside thereof
is substituted with an inert gas that does not contain moisture, by
using an air-tight sealing structure. In addition, another object
of the invention is to provide a thin-plate LN optical control
device in which drive voltage is low, an optical bandwidth is
broad, reliability is high, and mass productivity is excellent.
To accomplish the objects, a thin-plate LN optical control device
of the invention has the following technical characteristics.
(1) According to an aspect of the invention, there is provided
thin-plate LN optical control device including: a thin-plate LN
optical waveguide element which includes an optical waveguide
formed by thermal diffusion of Ti, and a control electrode
configured to control a light wave propagating through the optical
waveguide in a substrate made of lithium niobate, and in which at
least a part of the substrate is thinned; and a housing that
accommodates the thin-plate LN optical waveguide element in an
air-tight sealing manner. Oxygen is contained in a filler gas
inside the housing.
(2) In the thin-plate LN optical control device according to (1), a
molar concentration of oxygen in the filler gas may be 3% or
greater.
(3) In the thin-plate LN optical control device according to (1) or
(2), in the thin-plate LN optical waveguide element, the thickness
of the substrate may be 10 .mu.m or less.
(4) In the thin-plate LN optical control device according to (1) or
(2), in the thin-plate LN optical waveguide element, a ridge may be
formed in a front surface or a back surface of the substrate.
According to the aspect of the invention, in the thin-plate LN
optical control device including: the thin-plate LN optical
waveguide element which includes the optical waveguide formed by
thermal diffusion of Ti in the LN substrate, and the control
electrode that is formed on the substrate and is configured to
control a light wave propagating through the optical waveguide, and
in which at least a part of the substrate is thinned; and the
housing that accommodates the thin-plate LN optical waveguide
element in an air-tight sealing manner, since oxygen is contained
in the filler gas inside the housing, it is possible to suppress an
increase in DC drift amount. In addition, a variation between
samples is small, and thus satisfactory reproducibility is
obtained. As a result, it is possible to obtain a thin-plate LN
optical control device excellent in mass productivity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a thin-plate LN optical
waveguide element of a thin-plate LN optical modulator as one of
thin-plate LN optical control devices.
FIG. 2 is a graph illustrating a temporal variation of a DC drift
amount in a thin-plate LN optical modulator in which the thin-plate
LN optical waveguide element illustrated in FIG. 1 is accommodated
in a housing by using an air-tight sealing structure in a case
where an atmosphere inside the housing is switched from vacuum into
air.
FIG. 3 is a graph illustrating a temporal variation of an
inter-control-electrode current in a thin-plate LN optical
modulator in which the thin-plate LN optical waveguide element
illustrated in FIG. 1 is accommodated in a housing by using an
air-tight sealing structure in a case where an atmosphere inside
the housing is set to a nitrogen atmosphere that does not contain
moisture, and an air atmosphere that does not contain moisture.
FIG. 4A is a view illustrating a cross-sectional structure of Test
Specimen 1 that is used to specify the cause for occurrence of the
temporal variation of the inter-control-electrode current.
FIG. 4B is a view illustrating a cross-sectional structure of Test
Specimen 2 (subjected to Ti thermal diffusion treatment) that is
used to specify the cause for occurrence of the temporal variation
of the inter-control-electrode current.
FIG. 4C is a view illustrating a cross-sectional structure of Test
Specimen 3 (subjected to a polishing treatment) that is used to
specify the cause for occurrence of the temporal variation of the
inter-control-electrode current.
FIG. 5 is a graph illustrating the temporal variation of the
inter-electrode current in Test Specimen 1 illustrated in FIG. 4A
in a case of a nitrogen atmosphere that does not contain moisture
and in a case of an air atmosphere that does not contain
moisture.
FIG. 6 is a graph illustrating the temporal variation of the
inter-electrode current in Test Specimen 2 illustrated in FIG. 4B
in a case of a nitrogen atmosphere that does not contain moisture
and in a case of an air atmosphere that does not contain
moisture.
FIG. 7 is a graph illustrating the temporal variation of the
inter-electrode current in Test Specimen 3 illustrated in FIG. 4C
in a case of a nitrogen atmosphere that does not contain moisture
and in a case of an air atmosphere that does not contain
moisture.
FIG. 8 is a graph illustrating the temporal variation of the
inter-electrode current in Test Specimen 3 illustrated in FIG. 4C
in a nitrogen atmosphere that does not contain moisture in a case
of introducing oxygen not containing moisture on the way.
FIG. 9 is a graph illustrating an inter-electrode current value
with respect to a molar concentration of oxygen that does not
contain moisture in Test Specimen 3 illustrated in FIG. 4C.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a thin-plate LN optical control device of the
invention will be described in detail.
According to the invention, there is provided a thin-plate LN
optical control device including: a thin-plate LN optical waveguide
element which includes an optical waveguide formed by thermal
diffusion of Ti and a control electrode configured to control a
light wave propagating through the optical waveguide in a substrate
made of lithium niobate, and in which at least a part of the
substrate is thinned; and a housing that accommodates the
thin-plate LN optical waveguide element in an air-tight sealing
manner. Oxygen is contained in a filler gas inside the housing.
"Thinning" in the invention represents a state in which the
thickness of at least a part of a substrate is made to be small by
mechanical processing such as polishing and cutting. The "thinning"
includes not only a state in which the entirety of the substrate is
made to be thin but also a state in which a ridge is formed in a
front surface or a back surface of the substrate. In a case where
the thickness of a processed portion of the substrate is 30 .mu.m
or less and preferably 10 .mu.m or less, the invention is
effectively applied. In addition, in a case where a thinned portion
of the substrate exists in the vicinity of the optical waveguide or
at a site close to a substrate portion to which an electric field
is applied by the control electrode, and in a case where a
processing damage exists on a substrate surface that is exposed to
an atmosphere, an effect of the invention is relatively high. In
addition, even in a case of not performing a process of removing
the processing damage, which remains on the substrate due to the
mechanical processing, by chemicals or by thermal annealing, the
invention is preferably applied.
The present inventors have made a thorough investigation. As a
result, they obtained the following finding. In a thin-plate LN
optical modulator, in a case where a behavior of DC drift is
greatly different due to a filler gas inside the housing, and
oxygen is contained in the filler gas, an increase in
inter-control-electrode current value that is a parameter
corresponding to a DC drift amount is suppressed, and thus a
variation between samples decreases, and satisfactory
reproducibility is obtained. In addition, with regard to the filler
gas, it is preferable to use a gas that does not contain moisture
from the viewpoint of preventing a decrease in adhesion of an
adhesive, and disconnection or short-circuiting of an electric
interconnection.
First, the DC drift of the thin-plate LN optical modulator exhibits
a behavior that is different in accordance with an atmosphere
inside the housing. Accordingly, a thin-plate LN optical waveguide
element having a cross-sectional shape as illustrated in FIG. 1 is
prepared.
The thin-plate LN optical waveguide element is prepared as follows.
First, a Ti film that is patterned in a March-Zehnder structure is
formed on one surface of an X-cut LN substrate by using a
photolithography technology. Then, the Ti film is thermally
diffused in the LN substrate to form a Ti-diffused optical
waveguide.
A surface (back surface) opposite to the surface, in which the
Ti-diffused optical waveguide is formed, is polished so that the LN
substrate has a small plate thickness of 9 .mu.m. The LN substrate
is adhered to an X-cut LN substrate having a thickness of 500 .mu.m
for reinforcement by using an adhesive.
Next, the control electrode that controls a light wave propagating
through the Ti-diffused optical waveguide is formed as follows. As
a sheet layer, Ti and Au films are formed on the surface, in which
the Ti-diffused optical waveguide is formed, in this order by a
vacuum vapor deposition method. When forming the control electrode,
a semi-additive method and electrolysis gold plating are used.
With regard to the control electrode, a coplanar structure is used.
A signal electrode (an electrode at the center in FIG. 1) is formed
between arms of the Ti-diffused optical waveguide having the
Mach-Zehnder structure, and a ground electrode is formed on an
outer side of the arms. A gap between the signal electrode and the
ground electrode is set to 14 .mu.m. In addition, in the control
electrode, a length (in a direction perpendicular to a paper
surface in the drawing) of a portion that controls a light wave
propagating through the Ti-diffused optical waveguide is set to 10
mm. The thickness of the control electrode is set to 40 .mu.m.
Then, the resultant body is cut into chips.
The thin-plate LN optical waveguide element is fixed to the inside
of a stainless metal housing through an adhesive to be accommodated
therein, and an optical fiber is coupled to an optical waveguide
end on both end surfaces of the thin-plate LN optical waveguide
element by using butt joint. The housing is provided with a
structure capable of realizing introduction and substitution of a
gas. Specifically, a gas joint is provided in a lateral surface of
the housing. The housing, in which the thin-plate LN optical
waveguide element is accommodated, is sealed by seam-welding a lid
thereto. Furthermore, a structure and a method, which are capable
of realizing sealing, are also used an introduction portion of the
optical fiber into the housing.
A DC voltage in which an initial control voltage is set to 3.5 V is
applied to the control electrode of the thin-plate LN optical
modulator in which the thin-plate LN optical waveguide element
illustrated in FIG. 1 is accommodated in the housing at a test
environment temperature of 85.degree. C., and a temporal variation
of the control voltage is measured.
An atmosphere inside the housing is set to vacuum from initiation
of measurement to 17 hours, and the vacuum atmosphere is
substituted with a synthetic air (oxygen: 20%, nitrogen: 80%) that
does not contain moisture. FIG. 2 is a graph illustrating a
temporal variation of the control voltage of the thin-plate LN
optical modulator in the above-described atmosphere.
From the graph in FIG. 2, it can be seen that the control voltage
varies in a positive direction from 3.5 V to approximately 5.0 V
when the atmosphere inside the housing is vacuum, and the control
voltage rapidly varies in a negative direction immediately after
the atmosphere inside the housing is substituted with dry air. This
variation represents that DC drift of the thin-plate LN optical
modulator is strongly affected by the atmosphere inside the
housing.
The reason of atmosphere dependency of the DC drift of the
thin-plate LN optical modulator is assumed to be because electrical
resistance between the signal electrode and the ground electrode
varies with the elapse of time due to an atmosphere exposed to an
LN substrate surface on which the control electrode is formed, and
a variation (an absolute value of resistance or a time constant of
variation with the elapse of time) at this time is different.
Accordingly, for easiness of measurement, measurement of an
inter-control-electrode current (current that flows between control
electrodes (the signal electrode and the ground electrode) with the
optical waveguide interposed therebetween) is performed instead of
measurement of a temporal variation of the control voltage of the
thin-plate LN optical modulator.
First, a temporal variation of the inter-control-electrode current
is measured by using the thin-plate LN optical modulator with
respect to a case where the atmosphere inside the housing is set to
nitrogen that does not contain moisture and a case where the
atmosphere is set to air (synthetic air) that does not contain
moisture. A test environment temperature is set to 85.degree. C.,
and a voltage that is applied between control electrodes is set to
100 V. The number of times of measurement in the nitrogen
atmosphere is set to 12, and the number of times of measurement in
the synthetic air atmosphere is set to 5.
FIG. 3 is a graph illustrating a temporal variation of the
inter-control-electrode current of the thin-plate LN optical
modulator in the above-described atmosphere. In FIG. 3, a solid
line represents a temporal variation of the inter-control-electrode
current in a nitrogen atmosphere, a bold line represents an average
value, and fine lines on upper and lower sides represent "average
value+standard deviation (upper line) and "average value-standard
deviation (lower line). In addition, in FIG. 3, dotted lines
represent a temporal variation of the inter-control-electrode
current in the synthetic air atmosphere, and a bold line and fine
lines in the dotted lines represent the same things as in the
nitrogen atmosphere.
From the graph in FIG. 3, it can be seen that the
inter-control-electrode current in the synthetic air atmosphere is
smaller and a variation in the synthetic air atmosphere is smaller
in comparison to the nitrogen atmosphere.
As the conclusion for the dependency of the temporal variation of
the inter-control-electrode current on the atmosphere, it can be
assumed that an increase in the DC drift amount and a variation
thereof the nitrogen atmosphere are larger in comparison to the
synthetic air atmosphere.
Next, three kinds of test specimens illustrated in FIG. 4A to FIG.
4C are prepared to analyze the cause for dependency of the
inter-control-electrode current on the atmosphere, and a temporal
variation of an inter-electrode current (current that flows between
electrodes of test specimens in which the optical waveguide is not
formed) is measured under the same atmosphere inside the housing
and the same measurement conditions as those for obtaining the
results in FIG. 3.
In Test Specimen 1 in FIG. 4A, electrodes having the same structure
as in FIG. 1 are formed in an X-cut LN substrate having a thickness
of 1 mm.
In Test Specimen 2 in FIG. 4B, a Ti film having a thickness of 100
nm is formed on one surface of an X-cut LN substrate having a
thickness of 1 mm and is subjected to thermal diffusion, and the
same electrode structure as in Test Specimen 1 is formed.
In Test Specimen 3 in FIG. 4C, one surface of an X-cut LN substrate
having a thickness of 1 mm is polished by the method of polishing
the back surface in FIG. 1, and the same electrode structure as in
Test Specimen 1 is formed on a surface of the polished LN
substrate.
FIG. 5 to FIG. 7 are views illustrating a temporal variation of an
inter-electrode current in a case where an atmosphere inside
housings of Test Specimen 1, Test Specimen 2, and Test Specimen 3
is set to a nitrogen atmosphere that does not contain moisture, and
in a case where the atmosphere is set to synthetic air that does
not contain moisture.
From the graph in FIG. 5, it can be seen that the inter-electrode
current slightly fluctuates immediately after initiation of test,
but a variation stops after the fluctuation, and an approximately
constant state is maintained. In addition, a difference in the
inter-electrode current due to an atmosphere is hardly shown.
From the graph in FIG. 6, it can be seen that the inter-electrode
current gradually increases with the elapse of time. A temporal
variation of the inter-electrode current is different from that of
Test Specimen 1, but a difference in the inter-electrode current
due to an atmosphere is not shown.
From the graph in FIG. 7, it can be seen that the inter-electrode
current rapidly increases immediately after initiation of test in a
case of the nitrogen atmosphere, and the variation becomes constant
after the increase. In contrast, in a case of the synthetic air
atmosphere, it can be seen that the inter-electrode current
gradually decreases immediately after initiation of test, and the
variation becomes constant after the decrease. In addition, an
inter-electrode current value at elapsed time, at which the
variation of the inter-electrode current value becomes constant, is
greater in the nitrogen atmosphere in comparison to the air
atmosphere. In addition, in the air atmosphere, it can be seen that
the inter-electrode current value becomes approximately the same as
the inter-electrode current value in Test Specimen 1.
From the results, even in the thin-plate LN optical waveguide
element that is subjected to polishing, it is assumed that the DC
drift amount is likely to increase in the nitrogen atmosphere, and
the DC drift amount is likely to decrease in the synthetic air
atmosphere.
The inherent cause for the variation in the behavior of the DC
drift and the inter-control-electrode current due to the atmosphere
inside the housing is not specified at this time. However, it is
assumed that a band structure of an LN substrate surface varies due
to a polishing damage that occurs in the vicinity of the LN
substrate surface by a polishing treatment.
In a case where the variation of the band structure is unevenly
distributed on the LN substrate surface, it is considered that a
variation of a band gap, a non-uniform spatial charge, or a defect
level is caused to occur, and thus the inter-control-electrode
current value is influenced. In addition, in a case where the LN
substrate surface on which the polishing damage exists is exposed
to an atmosphere including oxygen, it is assumed that oxygen or an
oxygen ion is adsorbed to the spatial charge or the defect level,
thereby compensating a band structure that has varied.
In Test Specimen 3 in FIG. 4C, a polished surface is exposed to an
atmosphere inside the housing. On the other hand, in the thin-plate
LN optical waveguide element in FIG. 1, a polished surface (back
surface of the LN substrate) is not exposed to an atmosphere inside
the housing due to an adhesive layer, and thus it is considered
that the behavior of the DC drift and the inter-control-electrode
current do not depend on the atmosphere.
However, as illustrated in FIG. 2, a temporal variation of an
operation point depends on the atmosphere. The cause for the
dependency is assumed to be because a polishing damage occurred on
the back surface of the LN substrate due to polishing reaches the
vicinity of a surface (front surface), in which the optical
waveguide is formed, of the LN substrate.
Next, the following two tests are performed by using Test Specimen
3 with focus given to oxygen, which is a component of the filler
gas, from measurement results of FIG. 3, and FIG. 5 to FIG. 7.
First, a first test is a test of measuring a temporal variation of
an inter-electrode current due to a difference in a filler gas at a
test temperature of 85.degree. C. in a state in which a DC voltage
of 100 V is applied between electrodes of Test Specimen 3.
The filler gas is set to nitrogen at first, and oxygen is
introduced (an oxygen molar concentration: 23%) on the way. Then,
the filler gas is substituted with nitrogen. FIG. 8 is a graph
illustrating the measurement result.
As can be seen from FIG. 8, in the nitrogen atmosphere at first, as
is the case with a result in a nitrogen atmosphere in FIG. 7, it
can be seen that an inter-electrode current increases, and it
enters a normal state. It can be seen that the inter-electrode
current decreases when oxygen is subsequently introduced, and the
inter-electrode current increases when the atmosphere is
substituted with nitrogen.
From the test results, it can be seen that the inter-electrode
current decreases due to oxygen.
Accordingly, in the subsequent test, an investigation has been made
with respect to the dependency of the inter-electrode current value
on the oxygen molar concentration. The test is a test of measuring
the inter-electrode current value at a test temperature of
85.degree. C., in a state in which a DC voltage of 100 V is applied
between electrodes, and in an atmosphere of another oxygen molar
concentration as test conditions. Measurement of the
inter-electrode current value is measured after elapse of three
hours at which an inter-electrode current becomes approximately the
constant value from initiation of voltage application.
FIG. 9 is a graph illustrating the result. From FIG. 9, it can be
seen that as the oxygen molar concentration increases, the
inter-electrode current value decreases. In addition, it can be
seen that at an oxygen molar concentration of 3%, the
inter-electrode current becomes 10.sup.-11 (A) or less, and
particularly, at an oxygen concentration of 5% or greater, the
inter-electrode current becomes approximately constant value
regardless of the oxygen molar concentration.
As described above, in a structure in which the thin-plate LN
optical waveguide element, which is obtained by thinning the LN
optical waveguide element including the optical waveguide formed by
thermal diffusion of Ti in the substrate that uses LN, and the
control electrode that is formed in the substrate and is configured
to control a light wave propagating the optical waveguide, is
accommodated in the housing in an air-tight sealing manner, it can
be seen that when oxygen is contained in the filler gas inside the
housing, it is possible to effectively suppress the DC drift.
Particularly, in a case where the thickness of the thin-plate LN
optical waveguide element is made to be small to approximately 10
.mu.m by polishing, the effect of suppressing the DC drift due to
oxygen becomes significant. In the above-described tests,
description has been made with focus given to the polishing as
mechanical processing. However, even in plasma etching,
sandblasting, and the like when removing the vicinity of both sides
of the optical waveguide so as to allow the Ti-diffused optical
waveguide formed in the LN substrate to have a ridge structure, it
is assumed that a damage occurs on the LN substrate surface in the
same manner, and thus it is possible to expect suppression of the
DC drift due to oxygen.
In addition, as can be seen from FIG. 6, it is assumed that the DC
drift amount increases when considering that the inter-electrode
current value in a case where Ti is thermally diffused increases
from 10.sup.-11 (A) with the elapse of voltage application time.
Accordingly, in a case where mechanical processing is additionally
performed with respect to an LN optical waveguide element in which
an optical waveguide is formed by thermal diffusion of Ti, when the
invention is applied to the LN optical waveguide element, it is
possible to expect suppression of the DC drift.
In the examples, description has been given of the thin-plate LN
optical modulator as an example of a thin-plate LN optical control
device. However, it is needless to say that the invention
suppresses a DC drift amount, for example, in a directional coupler
or a thin-plate LN optical switch using a Y-branched structure.
As described above, according to the invention, it is possible to
provide a thin-plate LN optical control device in which occurrence
of DC drift is suppressed and mass productivity is excellent.
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